# Simple relay circuit help

#### Robkiller

May 21, 2014
9
So I feel that this is so simple yet I am not getting it done. I want to take a transistor and place it in a normally closed state to run a normally open relay on a 12vdc system. I want to turn the transistor off using positive voltage ( I am guessing by adding positive voltage to the base) (I can call for what ever positive voltage is needed). This will turn off the relay back to its normally open state. When power is taken away from the Base of the transistor the relay kicks back on.

Please any help would be great. I am new to posting to the forum but I am watching it all the time. I am new at this and am learning circuit basics still.

#### Robkiller

May 21, 2014
9
I guess I need to have a max of 5 volts to add to the base of the transistor to make it kick off the relay. I can get any volt I want.

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393

Here's a simple and common way to do what you want.

Q2 drives the relay coil. R3 provides base bias to turn Q2 ON. D1 is needed to protect Q2 against the "back EMF" generated by the relay coil when Q2 turns OFF.

When a positive voltage (more than about 2.5~3.0V with these component values) is applied to the IN connection, relative to the 0V rail, this voltage biases Q1 ON, making it conduct and pull its collector down to near 0V, removing the bias from Q2, so Q2 and the relay turn OFF.

#### Robkiller

May 21, 2014
9
View attachment 13128

Here's a simple and common way to do what you want.

Q2 drives the relay coil. R3 provides base bias to turn Q2 ON. D1 is needed to protect Q2 against the "back EMF" generated by the relay coil when Q2 turns OFF.

When a positive voltage (more than about 2.5~3.0V with these component values) is applied to the IN connection, relative to the 0V rail, this voltage biases Q1 ON, making it conduct and pull its collector down to near 0V, removing the bias from Q2, so Q2 and the relay turn OFF.
Wow I had the right idea I was working on it but without the correct resistors mine would not work. Thats the hard part with knowing what the component's should do but not the math to arange them to assist in doing the job right. Thanks so much I will try it.

#### Robkiller

May 21, 2014
9
Do you have any books or ways I can learn how to choose the right resistors and placement? I had that concept but I dont understand how to come up with the resistor placement or values?

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
It depends how (and how much) you want to learn.

To do those calculations for this circuit, you need a basic understanding of how a transistor operates - in terms of how it is controlled, and what its current gain means - and you need to understand voltage, current and resistance, and how they are related.

Basic electronics tutorials are a dime a dozen on the web and they will answer most simple questions in many common situations.

If you're more disciplined, and serious about learning, you can do a proper course, or at least read some books. One I've heard recommended is "Practical Electronics for Inventors" by Scherz and Monk.

Some kitsets also have tutorial-style explanations of how they work.

#### Robkiller

May 21, 2014
9
Thanks I will look into your recommendations.

#### Robkiller

May 21, 2014
9
So i finally got the parts to start building the circuit. I have attached a picture of what i have on a bread board. The relay is not turning on when 12v is applied to the circuit. Can you see why that may be?

#### Attachments

• circuit.jpg
204.9 KB · Views: 90

#### Robkiller

May 21, 2014
9
Got it had to switch the wire I had on ground for the relay to positive.

#### Robkiller

May 21, 2014
9
How many volts can the transistor handle on the 5v switch lead? Can a transistor be bad but show good on a volt meter. I am finding that the relay wont turn on until I take out the second transistor used to turn the relay off. I placed 12v on it temporarily to test it and it worked. The relay would turn off and on however I am wondering if I blue the 5v transistor and its stuck on the open position turning off the relay. Sorry for all the questions. I am learning

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
Your breadboard layout looks OK apart from the relay error which you fixed. Assuming you're using 2N3904 transistors. Also you have used 12k resistors instead of 3k3 resistors. The circuit will probably still work but you should change those resistors to 3k3.

With R1 = 10k, the circuit will withstand up to +30V on the input before R1 starts to get significantly warm. Q1 will not be damaged. The input voltage should not go more than about 8V negative as this WILL damage Q1.

A transistor can be bad but show good on a multimeter. It depends how you test it.

So the relay won't turn ON until you remove Q1 from the breadboard? And that's with the circuit input disconnected? That sounds like Q1 is damaged. Try removing Q2, then putting the transistor from the Q1 position into the Q2 position, except don't plug the base lead (middle lead) into the breadboard. If the relay clicks when you plug the collector and emitter into the board, that transistor is probably damaged.

#### Robkiller

May 21, 2014
9
Your breadboard layout looks OK apart from the relay error which you fixed. Assuming you're using 2N3904 transistors. Also you have used 12k resistors instead of 3k3 resistors. The circuit will probably still work but you should change those resistors to 3k3.

With R1 = 10k, the circuit will withstand up to +30V on the input before R1 starts to get significantly warm. Q1 will not be damaged. The input voltage should not go more than about 8V negative as this WILL damage Q1.

A transistor can be bad but show good on a multimeter. It depends how you test it.

So the relay won't turn ON until you remove Q1 from the breadboard? And that's with the circuit input disconnected? That sounds like Q1 is damaged. Try removing Q2, then putting the transistor from the Q1 position into the Q2 position, except don't plug the base lead (middle lead) into the breadboard. If the relay clicks when you plug the collector and emitter into the board, that transistor is probably damaged.

I will try this out thanks so much. The resistors I used says 3.3k on the package. I will also make sure they are and they were not miss labeled. I assume 3k3 is 3.3k is that correct? Thanks again.

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
Yes, 3k3 is 3.3k.

Those resistors are 12k. Their colour bands are brown, red, orange, gold. That translates to 12k.

3k3 resistors will have colour bands orange, orange, red, gold. Or if they're less than about 15 years old, they will probably have five bands, which should be orange, orange, black, brown, and something else.

Google resistor colour code (or resistor color code) for details.

#### BobK

Jan 5, 2010
7,682
Huh? I see two resistors with orange orange red and one with brown black orange. 3K3 and 10K as in your schematic.

Bob

#### KrisBlueNZ

##### Sadly passed away in 2015
Nov 28, 2011
8,393
Oh! You're right. I must have thought the gold bands were brown. Sorry!

#### BobK

Jan 5, 2010
7,682
Of course colors do look different on different monitors.

Bob

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